1. Field of the Invention
The present invention relates to a digital AGC circuit used in an image processing apparatus such as a facsimile or a digital copying machine for adjusting the dynamic range of an input signal in a digital manner.
2. Description of the Prior Art
In an image processing apparatus for optically reading an image and processing the image, for example, a facsimile or a copying machine, an image signal obtained by the reading has been conventionally subjected to AGC (Automatic Gain Control) processing so as to adjust the dynamic range of the image signal.
FIG. 9 is a block diagram for explaining one example of the electrical construction of an AGC circuit for realizing AGC processing. The AGC circuit 100 comprises an input interface unit 102, an analog-to-digital converting unit (hereinafter referred to as "A/D converting unit") 103 and an AGC processing unit 104. Image data outputted from a scanner 101 for reading an image of, for example, a CCD image sensor or a CIS image sensor is applied to the input interface unit 102. In the input interface unit 102, the applied image data is subjected to AGC processing. As a result, the dynamic range of the applied image data is adjusted.
Thereafter, the image data whose dynamic range is adjusted is applied to the A/D converting unit 103. In the A/D converting unit 103, the image data is converted into digital image data, after which the digital image data is applied to the AGC processing unit 104 for controlling the AGC processing in the input interface unit 102.
The AGC processing in the input interface unit 102 is performed on the basis of a gain control signal outputted from the AGC processing unit 104. More specifically, the input interface unit 102 comprises amplifying sections 102a and 102b in two stages for amplifying the image data outputted from the scanner 101 and a gain adjusting element 102c such as a variable resistive element for adjusting the gain of the amplifying section 102a in the front stage. The above described gain control signal is applied to the gain adjusting element 102c.
The gain adjusting element 102c adjusts, if it receives the gain control signal, the gain of the amplifying section 102a in the front stage in response to the gain control signal. As a result, the dynamic range of the image data outputted from the amplifying section 102a is adjusted.
The AGC processing is thus achieved.
In the above described AGC circuit 100, however, an analog element such as the gain adjusting element 102c is indispensable, resulting in a complicated circuit arrangement.
Furthermore, in the above described AGC circuit 100, the image data read by the scanner 101 is weak, thereby to make it extremely difficult to finely adjust the dynamic range. In the subsequent image processing, therefore, tone properties by density are degraded. On the other hand, an attempt to realize the fine adjustment requires a high-cost element, resulting in increased cost.